As a traditional wind turbine generator, there are a structure in which wind power rotates a propeller and a rotation of the propeller is directly sent to a direct-power generator, or sent to a generator via a gearbox having a prescribed speed increasing ratio, or a structure in which the rotation of the propeller is sent to a generator via a fluid type transmission which is a gearbox constituted of a hydraulic pump and a hydraulic motor. Patent Document 1, JP3822100B discloses one example of a fluid type transmission.
Further, more examples of using a fluid type transmission constituted of a hydraulic motor and a hydraulic pump as a gearbox are disclosed in Patent Document 2, US 2010/0032959A and also in Patent Document 3, US 2010/0040470A.
Patent Documents 2 and 3 propose the hydraulic pump mounted on the main shaft of the wind turbine, structured such that a plurality of cylinders are arranged in the radial direction of the main shaft and radially in a circumferential direction and pistons are arranged slidably in the cylinders such as to be moved by a relative rotation speed difference between the rotation of the main shaft and a rotation of a nacelle side.